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The selection method determines how individuals are chosed for mating. If you use a selection method that picks only the best individual, then the population will quickly converge to that individual. So the selector should be biased toward better individuals, but should also pick some that aren't quite as good (but hopefully have some good genetic material in them).

Some of the more common methods include roulette wheel selection (the likliehood of picking an individual is proportional to the individual's score), tournament selection (a number of individuals are picked using roulette wheel selection, then the best of these is (are) chosen for mating), and rank selection (pick the best individual every time). Threshold selection can also be effective.

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